There has been recently proposed, for example, a process control system in an industrial automation, which is configured by a wireless control network system using wireless communication. This wireless control network system is provided to solve drawbacks, caused by a conventional control system configured by a wired network, in that sensors for measuring temperatures and flow rates can not be installed at an optimum position in a plant due to the limitation of a communication distance and restriction of routing of wirings, and so forth, thereby deteriorating control accuracy.
Further, there has been also proposed a wireless control network management system for controlling the operation of the wireless control network system to optimize the operation of the plant in whole.
There is the following prior art reference relating to a conventional wireless control network management system.
[Patent Document 1] JP 2005-515695A
FIG. 7 is a block diagram showing a configuration of an example of a wireless control network management system using conventional gateway devices. In FIG. 7, the conventional wireless control network management system is made up of a host 1 for monitoring and controlling the system in whole by collecting and storing data from wireless nodes 41 to 48, gateway devices 2 and 3 (hereinafter referred to as GW devices, and expressed as GW in the drawings) for implementing communication with each wireless node, and the wireless nodes 41 to 48 (expressed as WN in the drawings) having various functions provided in field equipment such as a sensor's function for measuring physical quantity such as temperatures, flow rates, or an actuator's function for controlling a control valve, and a wireless communication function for implementing wireless transmission of measurement data.
That is, the wireless nodes 41 to 48 each have functions of various field equipment (hereinafter referred to as field equipment) such as a differential-pressure meter, a flow meter, a temperature instrument, an actuator and a controller.
The monitoring/controlling host 1 (hereinafter referred to as host) for implementing data communication with each wireless node is connected to GW devices 2 and 3 via an IP network NW 100 wherein data communication is implemented based on an IP protocol.
The GW device 2 is connected to wireless nodes 41 and 42 via a wireless network circuit (hereinafter referred to as wireless circuit), while the GW device 3 is connected to the wireless nodes 42 and 43 via the wireless circuit.
The wireless node 41 is connected to the wireless node 44 via the wireless circuit, and the wireless node 42 is connected to the wireless nodes 44 and 45 via the wireless circuit, while the wireless node 43 is connected to the wireless node 45 via the wireless circuit.
Further, the wireless node 44 is connected to the wireless nodes 46 and 47 via the wireless circuit, while the wireless node 45 is connected to the wireless nodes 47 and 48 via the wireless circuit.
Meanwhile, the wireless nodes 41 to 48 form a mesh-type multi-hop wireless network. That is, the GW devices 2 and 3 and the host 1 form a wired network while the GW devices 2 and 3, and the wireless nodes 41 to 48 form a wireless network.
The GW device 2 or 3, not shown in particular, is made up of wireless communication means for implementing wireless communication with the wireless nodes, communication means (wired) for implementing communication based on an IP protocol, between the host 1 and other GW device 3 or 2, an arithmetic control section for controlling operations of each means, and storage means for storing therein a program for causing the gateway device to operate as a gateway device and routing information reaching the host 1 from a self-device.
The wireless nodes 41 to 48 are made up of wireless communication means for implementing communication with other wireless nodes or GW device 3 or 2, an arithmetic control section for controlling operations of each means, and storage means for storing therein a program for causing the wireless nodes 41 to 48 to operate as wireless nodes, and routing information reaching the gateway device from each wireless node.
Here, the wireless nodes 41 to 48 figure out in advance routing information for transferring data to the GW device 2 or 3 by implementing route search together with address/name resolution, or setting up in advance the routing information on each wireless node by an operator.
Further, the GW device 2 or 3 figures out in advance routing information reaching each one of wireless nodes 41 to 48 or the host 1 by implementing route search together with address/name resolution, or setting up in advance the routing information on each wireless node.
FIG. 8 is a view for explaining data communication between the host 1 and the wireless node 47 in the conventional wireless control network management system, and the explanation of operations (explanation in each step) is described in FIG. 8.
In step SP101, the host 1 implements resolution of route addressed to the wireless node 47 based on routing information. That is, the host 1 selects the gateway device (e.g. GW device 2) through which data passes from the host 1. Although there are means for specifying a default gateway by an operator and means for using ARP (Address Resolution Protocol) employed by an IP network for selecting the gateway device, any means may be employed.
In Step SP102, the host 1 uses an IP protocol and sends data to the GW device 2 via an IP network NW 100.
In Step SP 103, the GW device 2 implements conversion of communication protocol, for example, from data construction for IP protocol into data for mesh protocol employed by a wireless network so as to implement data communication with the wireless node 47 by sending data received from the host 1 via the IP network NW 100 by wireless communication means, not shown.
In Step SP104, the GW device 2 implements optimum routing so as to send data to the wireless node 47 based on the routing information, and sends the data to the wireless node 41 on the mesh network via the wireless circuit. Meanwhile, ZigBee or SA100.11a are illustrated by a typical example of a mesh routing technique (routing technique).
In Step SP105, the wireless node 41 implements optimum routing so as to send data received from the GW device 2 to the wireless node 47 based on the routing information, and transfers data to the wireless node 44 via the wireless circuit.
In Step SP106, the wireless node 44 sends data received from the wireless node 41 to the wireless node 47 based on the routing information via the wireless circuit.
In such a manner, according to the wireless control network management system using the conventional gateway devices, data communication between the host 1 and each wireless node can be implemented, so that, for example, the host 1 can implement send/receive of control data for adjusting valve opening of a control valve to a wireless node having an actuator's function, thereby supporting an optimum operation of a plant.
However, according to the conventional gateway devices and the wireless control network management system using the same, the wireless communication has an error factor inherent thereto such as obstruction and noise. If such communication failure occurs on a mesh network, there was a problem of the increase of a possibility of data loss between the host 1, the GW devices 2 and 3 and the wireless nodes 41 to 48.
Further, there is proposed a technique capable of avoiding the foregoing communication failure by use of routing redundancy of the mesh network. However, even in such a technique, there still exists a problem in that an error cannot be avoided if there occurs wireless communication failure at a large area or at the periphery of the gateway devices.
Further, a distance of a mesh route, i.e. a physical distance between wireless nodes or the number of hops increases depending on the positional relation between the gateway device selected by the host 1 and the wireless node serving as a data destination, causing a problem of the increase of a possibility of occurrence of a wireless communication error.
Still further, there is proposed a technique of implementing optimum routing, for example, by updating a mesh route up to the minute when the host implements resolution of selection of the gateway devices in order to avoid the foregoing problems. However, even in such a technique, the update of the mesh route generally requires data exchange such as broadcast of a control packet or notice of information of a communication link, and if data exchange is implemented excessively, leading to a problem of invitation of flooding of packets or the increase of a possibility of causing communication failure such as increase in load or radio interference caused by congestion.
Although it is possible to mitigate the impacts caused by congestion by adjusting frequency of routing update, communication environment of the wireless network is varied in response to the change of a physical condition or movement of wireless nodes, leading to a problem in that the optimum routing becomes difficult to be maintained depending on the frequency of routing update.